This invention relates in general to shifting mechanisms for power take-offs. In particular, this invention relates to an improved structure for a hydraulically operated shifter for use in such a power take-off.
Power take-offs are well known mechanical devices that are commonly used in conjunction with sources of rotational energy, such as engines and transmissions contained in vehicles, for selectively providing power to one or more rotatably driven accessories. For example, power take-offs are commonly used in a variety of industrial and agricultural vehicles for operating hydraulic pumps that, in turn, operate hydraulically driven accessories, such as plows, trash compactors, lifting mechanisms, winches, and the like. The power take-off provides a relatively simple and inexpensive mechanism for supplying rotational power from the source of rotational energy to operate the rotatably driven accessory.
A typical power take-off includes a hollow housing having a mounting surface provided thereon. An opening is formed through the mounting surface of the power take-off housing. An input gear is rotatably supported within the power take-off housing and includes a portion that extends outwardly through the opening formed through the mounting surface. The mounting surface of the power take-off housing is adapted to be secured (typically by a plurality of bolts) to a corresponding mounting surface provided on a case of the source of rotational power, such as a vehicle transmission or engine. The mounting surface provided on the transmission case also has an opening formed therethrough. When the power take-off housing is secured to the transmission case, the opening formed through the mounting surface of the power take-off housing is aligned with the opening formed through the transmission case. This allows the input gear of the power take-off to extend through such aligned openings into meshing engagement with one of the gears contained within the transmission. Typically, the input gear of the power take-off meshes with a transmission gear that is constantly driven by the vehicle engine. As a result, the input gear of the power take-off is rotatably driven by the transmission gear whenever the vehicle engine is operated.
The power take-off further includes an output shaft that is rotatably supported within the power take-off housing. A portion of the output shaft extends outwardly from the power take-off housing and is adapted to be connected to the rotatably driven accessory. In some power take-offs, the output shaft is constantly connected for rotation by the input gear. In those instances, the output shaft rotatably drives the rotatably driven accessory whenever the input gear is rotatably driven by the transmission gear. In other power take-offs, however, the output shaft is only intermittently connected for rotation by the input gear by means of an intermediate clutch. When the clutch connects the output shaft for rotation by the input gear, the output shaft rotatably drives the rotatably driven accessory whenever the input gear is rotatably driven by the transmission gear. When the clutch disconnects the output shaft from rotation by the input gear, the output shaft does not rotatably drive the rotatably driven accessory.
This engagement and disengagement of the clutch is controlled by a shifter that is typically provided on the housing of the power take-off. A typical shifter includes a hydraulic or pneumatic piston and cylinder assembly that is connected to a movable shift fork. Frequently, the shift fork is connected to the piston such that movement of the piston within the cylinder causes movement of the shift fork. When the piston and cylinder assembly is operated in a first mode, the piston and the shift fork are moved in a first direction. This movement in the first direction causes the clutch to become disengaged, thereby disconnecting the output shaft from rotation by the input gear and preventing operation of the rotatably driven accessory. When the piston and cylinder assembly is operated in a second mode, the piston and the shift fork are moved in a second direction. This movement in the second direction causes the clutch to become engaged, thereby connecting the output shaft for rotation by the input gear and causing operation of the rotatably driven accessory.
A biasing mechanism, such as a spring, may be provided within the shifter to urge the piston and the shift fork for movement in the first direction. When so provided, the biasing mechanism normally maintains the clutch of the power take-off in a first operating condition (either engaged or disengaged as desired). To operate the clutch in a second operating condition (either disengaged or engaged, respectively), pressurized fluid is supplied within the piston and cylinder assembly. This pressurized fluid urges the piston and the shift fork for movement in the second direction against the urging of the biasing mechanism. By controlling the application of this pressurized fluid, the clutch of the power take-off can be operated as desired.
In order to control the application of this pressurized fluid in this manner, a fluid control valve is usually provided. Typically, the fluid control valve is embodied as a solenoid valve that includes an inlet port, an outlet port, and an exhaust port. The inlet port of the control valve communicates with a source of pressurized fluid, such as a pump or a compressor that may be provided on or within the transmission. The outlet port of the control valve communicates with the piston and cylinder assembly. The exhaust port of the control valve communicates with a reservoir of the fluid, such as may be provided on or within the transmission. When the control valve is actuated in a first operating condition, fluid communication is prevented between the inlet port and the outlet port, while fluid communication permitted is between the exhaust port and the outlet port. As a result, pressurized fluid is not supplied within the piston and cylinder assembly, and the piston and cylinder assembly is vented to the reservoir of the fluid. Consequently, the clutch of the power take-off is operated in the first operating condition described above. When the control valve is actuated in a second operating condition, fluid communication is permitted between the inlet port and the outlet port, while fluid communication is prevented between the exhaust port and the outlet port. As a result, pressurized fluid is supplied within the piston and cylinder assembly, and the piston and cylinder assembly is not vented to the reservoir of the fluid. Consequently, the clutch of the power take-off is operated in the second operating condition described above.
When the shifter is operated pneumatically (i.e., by means of a gaseous medium, such as air), the exhaust port of the control valve can be simply vented to the atmosphere, which essentially functions as the reservoir of the fluid. However, when the shifter is operated hydraulically (i.e., by means of a liquid medium, such as oil), a liquid return line is provided between the exhaust port of the control valve and the reservoir of the fluid, which is usually located within the power take-off or the transmission. Although the use of such a liquid return line has functioned satisfactorily, it has been found that the need for providing such a liquid return line from the exhaust port of the control valve and the reservoir of the fluid increases the complexity and cost of the shifter. Thus, it would be desirable to provide an improved structure for a hydraulically operated shifter for use in a power take-off that is simpler and less expensive in construction than known structures.